JP6138877B2 - Amplifying optical fiber and optical fiber amplifier using the same - Google Patents

Description

The present invention relates to an amplification optical fiber used for optical communication and an optical fiber amplifier using the same, and is suitable for suppressing the difference between the gain of LP ₀₁ mode light and the gain of LP ₁₁ mode light. is there.

In optical communication, communication is performed by superimposing a signal on light propagating through an optical fiber. In order to increase the amount of information transmitted by one core in such optical communication, information is superimposed on the LP ₀₁ mode (basic mode) light in the signal light and information is superimposed on the LP ₁₁ mode light. Fu-mode communication for performing communication is known. Therefore, when amplifying signal light propagating through an optical fiber using an optical fiber amplifier, it is necessary to amplify both LP ₀₁ mode light and LP ₀₂ mode light.

Non-Patent Document 1 below describes such an amplification optical fiber. In the optical fiber for amplification described in Non-Patent Document 1 below, the refractive index of the core is a step type, that is, the refractive index in the core is constant in the radial direction. In addition, erbium is added to the core, and the concentration of erbium is constant in the core. However, when amplifying light using such an amplification optical fiber, there is a tendency that a difference occurs between the gain of the LP ₀₁ mode light and the gain of the LP ₁₁ mode light.

Therefore, the following Non-Patent Document 2 describes an amplification optical fiber for the purpose of suppressing such a gain difference. The amplification optical fiber described in Non-Patent Document 2 has a core refractive index distribution similar to that of the amplification optical fiber of Non-Patent Document 1, but erbium is not added in the central region including the central axis of the core. And erbium is added to the outer peripheral side region surrounding the central side region. Thus, the LP ₀₁ mode light power propagating in the erbium-doped region and the LP ₁₁ mode light power are aligned, and the LP ₀₁ mode light gain and the LP ₁₁ mode light gain are aligned. .

Y. Yung et al. , “First demonstration of multimode amplifier for spatial division multiplexed transmission systems,” Proc. ECOC '11, Th. 13. K4 (2011) G. Le Cocq et al. , “Modeling and charac- terization of fare-mode EDFA supporting four-mode division for multiplexing,” Opt. Express 20, 27051-27061 (2012)

However, even with the amplification optical fiber described in Non-Patent Document 2, the gain may vary between the LP ₀₁ mode light and the LP ₁₁ mode light. The difference in gain between the light beams in each mode is caused by the fact that the degree of excitation of erbium is not constant in the region where erbium is added. Excitation light for exciting erbium propagates through the core as multimode light in the amplification optical fiber. For this reason, in the area | region where the erbium in a core is added, the power of excitation light is not distributed uniformly. Therefore, the erbium inversion distribution is not constant in the region where erbium is added. Since the distribution of the erbium in the excited state is unevenly distributed, the power of the LP ₀₁ mode light of the signal light propagating through the region to which erbium is added is the same as the power of the LP ₁₁ mode light. However, the gain changes between the LP ₀₁ mode light and the LP ₁₁ mode light as described above.

Therefore, it is important to adjust the power of each mode of the pumping light propagating through the amplification optical fiber, that is, to adjust the excitation ratio of each mode in the pumping light. However, it is known that a complex optical system is required to adjust the excitation ratio, and a more complex optical system is required when the number of modes is large. Therefore, there is a demand for reducing the number of modes of the pumping light and easily suppressing the difference between the gain of the LP ₀₁ mode light of the signal light and the gain of the LP ₁₁ mode light.

Accordingly, an object of the present invention is to provide an amplifying optical fiber that can easily suppress the difference between the gain of the LP ₀₁ mode light and the gain of the LP ₁₁ mode light, and an optical fiber amplifier using the same. And

  In order to solve the above-mentioned problems, the present invention is an optical fiber for amplification comprising a core having an inner core and an outer core surrounding the outer peripheral surface of the inner core, and is characterized by the following.

That is, the relative refractive index difference of the inner core with respect to the cladding is smaller than the relative refractive index difference of the outer core with respect to the cladding, and erbium is added to the entire outer core, so that the theoretical cutoff wavelength of LP ₁₁ mode light is obtained. Is 1565 nm or more, the theoretical cutoff wavelength of LP ₂₁ mode light is 1530 nm or less, and the theoretical cutoff wavelength of LP ₀₂ mode light is 980 nm or less.

According to the amplification optical fiber of the present invention having the above characteristics, LP ₀₁ mode light and LP ₁₁ mode light can be amplified in a band in a wavelength range of 1530 nm to 1565 nm, that is, a C band band. Therefore, this amplification optical fiber can be used for an optical fiber amplifier for fu mode communication in which signal light is in the C band.

Further, since the relative refractive index difference with respect to the clad of the inner core is smaller than the relative refractive index difference with respect to the clad of the outer core, the power of the LP ₀₁ mode light can be unevenly distributed from the outer core. Moreover, since erbium is added to the outer core, when signal light propagates through the core, the power of the LP ₀₁ mode light and the power of the LP ₁₁ mode light propagating through the core erbium-doped region. Can be reduced. Further, since the theoretical cutoff wavelength of the LP ₀₂ mode is 980 nm or less, when the excitation light that excites erbium propagates through the core, the modes of the excitation light are changed to the LP ₀₁ mode, the LP ₁₁ mode, the LP ₂₁ mode, and the LP ₂₁ mode. It can be limited to ₃₁ modes. The number of modes of the excitation light can be reduced as compared with the existence of 6 modes of the excitation light in Non-Patent Document 2. Therefore, the excitation ratios of the light in the four modes may be controlled so that the gain of the LP ₀₁ mode light and the gain of the LP ₁₁ mode light in the signal light are equal. Therefore, according to the amplification optical fiber of the present invention, the difference between the gain of the LP ₀₁ mode light and the gain of the LP ₁₁ mode light can be easily suppressed.

Further, when the inner core diameter is D ₁ and the outer core outer diameter is D ₂ , D ₁ / D ₂ is 0.5 or more, and the relative refractive index difference between the inner core and the clad is delta and _nci, the relative refractive index difference with respect to the cladding of the outer core in the case of a Δ _nco, Δ _nci / Δ _nco is preferred to be 0.1 or less.

By configuring the core in this way, the power of the LP ₀₁ mode propagating through the outer core and the power of the LP ₁₁ mode light are generally set within a predetermined range of the LP ₂₁ mode light cutoff wavelength. Can be aligned. In addition, the theoretical cutoff wavelength of the LP ₀₂ mode light can be more appropriately set to 980 nm or less. It is obvious that D ₁ / D ₂ is smaller than 1 from the definition of the inner core and the outer core.

In this _case, it is more preferred that D 1 / _{D 2} is 0.6 or more.

By setting D ₁ / D _{2 to} be 0.6 or more, the difference between the power of the LP ₀₁ mode light propagating through the outer core and the power of the LP ₁₁ mode light can be made substantially zero.

In these cases, it is preferable that D ₁ / D ₂ is 0.8 or less.

In a region where D ₁ / D ₂ is larger than 0.8, the total power propagating through the outer core of the LP ₀₁ mode light of the signal light and the power propagating through the outer core of the LP ₁₁ mode light of the signal light The difference from the sum of the values does not change greatly compared to 0.8 or less. On the other hand, if D ₁ / D ₂ is 0.8 or less, it is possible to suppress a decrease in the gain of the LP ₀₁ mode light and the gain of the LP ₁₁ mode light in the signal light.

In these cases, Δ _nci / Δ _nco is preferably 0 or more.

The theoretical cutoff wavelength of LP ₂₁ mode light is preferably 1430 nm or more, and more preferably, the cutoff wavelength is 1450 nm or more.

  An optical fiber amplifier according to the present invention includes any one of the above-described amplification optical fibers and a pumping light source that emits pumping light having a wavelength of 980 nm that is incident on the core. is there.

This optical fiber amplifier can suppress the difference between the gain of the LP ₀₁ mode light and the gain of the LP ₁₁ mode light in the C-band. Therefore, the fuse mode communication in the C band can be performed more appropriately.

As described above, according to the present invention, there is provided an amplification optical fiber that can easily suppress the difference between the gain of LP ₀₁ mode light and the gain of LP ₁₁ mode light, and an optical fiber amplifier using the same. Provided.

It is a figure which shows the mode of a cross section perpendicular | vertical to the longitudinal direction of the optical fiber for amplification which concerns on embodiment of this invention. It is a figure which shows the mode of the relative refractive index difference of the optical fiber of FIG. 1, the concentration distribution of erbium, and the power distribution of light. The ratio of the outer diameter of the diameter and the outer core of the inner core is a diagram showing the relationship between the LP ₀₁ mode of the power and LP ₁₁ mode optical power of light propagating through the outer core. The figure which shows the relationship between the ratio of the relative refractive index difference with respect to the clad of the inner core and the relative refractive index difference with respect to the clad of the outer core, and the power of the LP ₀₁ mode light propagating through the outer core and the power of the LP ₁₁ mode light. It is. Is a diagram showing the relationship between the LP ₀₁ mode of the power and LP ₁₁ modes of light power of the light propagating the relative refractive index difference and the outer core with respect to the cladding of the outer core. It is a figure which shows the relationship between the mode which the light of wavelength 1550nm propagates, the radius of an inner core, and the outer periphery radius of an outer core. It illustrates a mode between gain differential between _{LP 01} mode of light and _{LP 11} mode of light at each point A~O in FIG. The difference between the LP ₀₁ mode optical power and LP ₁₁ modes of the power of light propagating in the outer core is a diagram showing the relationship between the inter-mode gain difference between the LP ₀₁ mode of light and LP ₁₁ mode of light. The ratio of LP ₀₁ mode optical power and LP ₁₁ modes of the power of light propagating in the outer core is a diagram showing the relationship between the inter-mode gain difference between the LP ₀₁ mode of light and LP ₁₁ mode of light. And the difference between the LP ₀₁ mode optical power and LP ₁₁ mode of light power of a diagram showing the relationship between the light cut-off wavelength of the LP ₂₁ mode. It is a figure which shows the relationship between the ratio of the diameter of an inner core and the outer diameter of an outer core, and a cutoff wavelength. It is a figure which shows the relationship between the ratio of the relative refractive index difference with respect to the clad of the inner core and the relative refractive index difference with respect to the clad of the outer core, and the cutoff wavelength. It is a figure which shows the optical fiber amplifier which concerns on this embodiment of this invention.

  Preferred embodiments of an amplification optical fiber and an optical fiber amplifier using the same according to the present invention will be described below in detail with reference to the drawings. For ease of understanding, the scale of each figure may be different from the scale described in the following description.

<Description of optical fiber for amplification>
FIG. 1 is a diagram showing a state of a cross section perpendicular to the longitudinal direction of an amplification optical fiber according to an embodiment of the present invention. As shown in FIG. 1, the amplification optical fiber 10 includes a core 11, a clad 12 that surrounds the outer peripheral surface of the core 11 without a gap, and a coating layer 14 that covers the clad 12. The diameter of the core 11 is, for example, 10 μm. Further, the outer diameter of the clad 12 is, for example, 125 μm.

The amplification optical fiber 10 of the present embodiment is a fu-mode fiber that propagates LP ₀₁ mode light and LP ₀₂ mode light in the C-band. That is, in the amplification optical fiber 10, the theoretical cutoff wavelength of the LP ₁₁ mode light is 1565 nm or more, and the theoretical cutoff wavelength of the LP ₂₁ mode light is 1530 nm or less.

FIG. 2 is a diagram illustrating the state of the core 11 of the amplification optical fiber 10 of FIG. 1 and the surroundings thereof. Specifically, FIG. 2A shows the core 11 and the clad 12 in the region indicated by the dotted line in FIG. 1, and FIG. 2B shows the refractive index distribution in the region shown in FIG. 2 (C) shows the concentration distribution of erbium added to the core, and FIG. 2 (D) shows the power distribution of the LP ₀₁ mode light and the LP ₀₂ mode light propagating through the core 11.

  As shown in FIG. 2A, the core 11 includes an inner core 11i including a central axis and an outer core 11o surrounding the outer peripheral surface of the inner core 11i without a gap.

  Further, as shown in FIG. 2B, the refractive index of the inner core 11i is lower than the refractive index of the outer core 11o. In the present embodiment, the refractive index of the inner core 11 i is equal to the refractive index of the cladding 12. The relative refractive index difference between the outer core 11o and the clad 12 is, for example, 1%. Because of having such a refractive index distribution, for example, the outer core 11o is made of quartz to which a dopant that increases the refractive index, such as germanium (Ge), is added, and the inner core 11i and the cladding 12 are made of quartz to which no dopant is added. Become. When the outer core 11o is made of quartz to which no dopant is added, the inner core 11i and the clad 12 are made of quartz to which a dopant that lowers the refractive index such as fluorine is added.

  Further, as shown in FIG. 2C, erbium is added to the outer core 11o. In this embodiment, erbium is added to the entire outer core 11o, and erbium is not added to the inner core 11i.

The amplification optical fiber 10 propagates light in the LP ₀₁ mode and the LP ₁₁ mode in the C band. When the refractive index of the core 11 is constant in the radial direction, the power peak of the LP ₀₁ mode light is originally located at the center of the core. However, since the core 11 of the amplification optical fiber 10 of this embodiment has a refractive index distribution as shown in FIG. 2B, the light propagating through the core is shifted to the outer peripheral side. Therefore, as shown in FIG. 2D, the LP ₀₁ mode light is also shifted to the outer peripheral side, and the power peak of the light is shifted from the center to the outer peripheral side. Further, the LP ₁₁ mode light is also shifted to the outer peripheral side. In the present embodiment, among the C-band light propagating through the core 11, the total power propagating through the outer core 11o out of the LP ₀₁ mode light and the outer core 11o out of the LP ₁₁ mode light are propagated. The sum of the power to be performed is almost equal.

Next, in the outer core 11o, a configuration in which the total power of the LP ₀₁ mode light and the total power of the LP ₁₁ mode light are approximately equal will be described.

3, the ratio of _{D 2} (diameter of the core 11) the inner core 11i diameter _{D 1} and the outer diameter of the outer core 11o, the _{LP 01} mode of light propagating in the outer core 11o power and _{LP 11} mode optical It is a figure which shows the relationship with the power of. In FIG. 3, the total power propagating through the outer core 11 o among the LP ₀₁ mode light is denoted by Γ ₀₁ , and the total power propagating through the outer core ₁₁ o among the LP ₁₁ mode light is denoted by Γ _11. Yes. The difference between Γ ₀₁ and Γ ₁₁ is indicated by ΔΓ. The vertical axis indicates the magnitudes of Γ ₀₁ , Γ ₁₁ , and ΔΓ, and the horizontal axis indicates the ratio D ₁ / D ₂ between the diameter D ₁ of the inner core 11 i and the outer diameter D ₂ of the outer core 11 o. Show. In FIG. 3, the LP ₂₁ mode light has a cutoff wavelength of 1450 nm, the relative refractive index difference with respect to the cladding 12 of the outer core 11o is 1%, and the refractive index of the inner core 11i is equivalent to that of the cladding. It defines a D _2. As shown in FIG. 3, when D ₁ / D ₂ is 0.5 or more, there is almost no difference in magnitude between Γ ₀₁ and Γ _11, and the absolute value of ΔΓ is 0.01 or less. Further, when D ₁ / D ₂ is 0.6 or more, the difference ΔΓ in magnitude between Γ ₀₁ and Γ ₁₁ is substantially zero. In addition, the upper limit of D ₁ / D ₂ is set to 0.8 so that the gain of the LP ₀₁ mode light and the gain of the LP ₁₁ mode light do not become too small because Γ ₀₁ and Γ ₁₁ become too small. It is preferable. In FIG. 3, the wavelength of light propagating through the core 11 is 1550 nm. However, the tendency as shown in FIG. Furthermore, since the cutoff wavelength of the light _{LP 21} mode as described above and 1450 nm, the cutoff wavelength of the light of at least _{LP 21} mode is 1450 _nm, the above trend of D 1 / _{D 2} can be said. However, even if the cutoff wavelength of the LP ₂₁ mode light is changed by about 20 nm from FIG. 3 as will be described later, the tendency is not so different from FIG. Therefore, if the cut-off wavelength of the LP ₂₁ mode light is 1430 nm, the tendency is not so different from FIG.

Next, when D ₁ / D ₂ is 0.5, the ratio Δ _nci / of the relative refractive index difference Δ _nci of the inner core _{11 i} to the cladding 12 and the relative refractive index difference Δ _nco of the outer core _{11 o} to the cladding 12. _Δnco is changed. FIG. 4 is a diagram showing the relationship between the ratio Δ _nci / Δ _nco and the power Γ ₀₁ of the LP ₀₁ mode light propagating through the outer core _{11 o} and the power Γ ₁₁ of the LP ₁₁ mode light. As shown in FIG. 4, when the ratio Δ _nci / Δ _nco is 0.1 or less, there is no difference in magnitude between Γ ₀₁ and Γ ₁₁ . 4 is the same as the condition of the wavelength propagating through the core 11 in FIG. 3, but the tendency of FIG. 4 does not change substantially at other wavelengths. Further, the condition of the cutoff wavelength of the LP ₂₁ mode light in FIG. 4 is the same as the condition of the cutoff wavelength of the LP ₂₁ mode light in FIG. Therefore, the above tendency of Δ _nci / Δ _nco can be said at least when the cut-off wavelength of the LP ₂₁ mode light is 1450 nm. Further, even if the cutoff wavelength of the LP ₂₁ mode light is changed by about 20 nm from FIG. 4, the tendency is not so different from FIG. Therefore, if the cut-off wavelength of the LP ₂₁ mode light is 1430 nm, the tendency is not so different from FIG.

FIG. 5 is a diagram showing the relationship between the relative refractive index difference Δ _nco of the outer core 11o with respect to the cladding 12 and the power Γ ₀₁ of the LP ₀₁ mode light propagating through the outer core 11o and the power Γ ₁₁ of the LP ₁₁ mode light. is there. In FIG. 5, the ratio D ₁ / D ₂ between the diameter D ₁ of the inner core 11 i and the outer diameter D ₂ of the outer core 11 o is 0.4, and the relative refractive index difference Δ _nci with respect to the cladding of the inner core 11 _i is zero. There, the cutoff wavelength of the light _{LP 21} mode is the outer diameter _{D 2} of the outer core 11o to be 1450 nm. Under such conditions, the horizontal axis represents the relative refractive index difference Δ _nco with respect to the cladding of the outer core 11o, and the vertical axis represents the power Γ ₀₁ of the LP ₀₁ mode light propagating through the outer core 11o and the light of the LP ₁₁ mode. The power Γ ₁₁ is shown. From FIG. 6, the difference ΔΓ between Γ ₀₁ and Γ ₁₁ is constant regardless of Δ _nco . That is, even if the refractive index of the outer core 11o changes, the change does not affect ΔΓ.

From FIG. 3 to FIG. 5, when D ₁ / D ₂ is 0.5 or more and Δ _nci / Δ _nco is 0.1 or less, LP ₀₁ mode light powers Γ ₀₁ and LP in the outer core 11o are obtained. _{The 11-} mode light power Γ ₁₁ is made substantially equal.

  Next, the relationship between the inter-mode gain difference (DMG) and the cutoff wavelength will be described with reference to FIGS.

FIG. 6 is a diagram illustrating a relationship between a mode in which light having a wavelength of 1550 nm propagates and the radius R _i of the inner core 11 _i and the radius R _o of the outer periphery of the outer core 11 _o . In FIG. 6, the wavelength of light propagating through the core 11 is 1550 nm, the relative refractive index difference Δ _nci of the inner core 11i with respect to the cladding 12 is 0%, and the relative refractive index difference Δ _nco of the outer core 11o with respect to the cladding 12 is 1%. It was. In Figure 6, by fixing the respective relative refractive index difference as described above, since the varying the radius R _o of radius R _i and an outer periphery of the outer core 11o of the inner core 11i, LP ₂₁ mode within 6 The cutoff wavelength of light is not constant.

6, the region AR _N diameter of the inner core 11i is greater than the outer diameter of the outer core 11o, an area impossible to physically. In addition, the region AR ₀ is an area where light is also not the propagation of any mode. The area AR ₁ is an area where only the LP ₀₁ mode light propagates. The area AR ₂ is an area where only LP ₀₁ mode light and LP ₁₁ mode light propagate. Region AR ₃ is a region where only LP ₀₁ mode light, LP ₁₁ mode light, and LP ₂₁ mode light propagate. The area AR ₄ is an area where only LP ₀₁ mode light, LP ₁₁ mode light, LP ₂₁ mode light, and LP ₀₂ mode light propagate. Therefore, the relationship between the radius R _i of the inner core 11 _i and the radius R _o of the outer periphery of the outer core 11 _o of the amplification optical fiber 10 of this embodiment needs to be in the area AR ₂ .

Next, the inter-mode gain difference at each point in the region AR _2, will be described. FIG. 7 is a diagram illustrating a difference in mode gain between the LP ₀₁ mode light and the LP ₁₁ mode light at the points A to O in the area AR ₂ in FIG. FIG. 8 shows the difference between the power Γ ₀₁ of the LP ₀₁ mode light propagating through the outer core 11o and the power Γ ₁₁ of the LP ₁₁ mode light (Γ ₀₁ −Γ ₁₁ ), the light of the LP ₀₁ mode and the LP ₁₁ mode. FIG. 9 is a graph showing the relationship between the mode gain and the mode gain difference, and FIG. 9 is a graph showing the ratio of the LP ₀₁ mode light power Γ ₀₁ and the LP ₁₁ mode light power Γ ₁₁ propagating through the outer core 11o ( and Γ ₁₁ / Γ _01), is a diagram showing the relationship between the inter-mode gain difference between the _{LP 01} mode of light and _{LP 11} mode of light. 7 to 9, the wavelength of light propagating through the core 11, the relative refractive index difference Δ _{nci with} respect to the cladding 12 of the inner core 11i, and the relative refractive index difference Δ _nco with respect to the cladding 12 of the outer core 11o are _set as the conditions in FIG. Same as above. 7 to 9, the gain difference between modes due to the LP ₀₁ mode pump light, the gain difference between modes due to the LP ₁₁ mode pump light, and the gain mode due to the LP ₂₁ mode pump light. The gain difference is shown individually.

As shown in FIG. 7, at the point E and the point F, it can be seen that the gain difference between modes is very small. The points E and F are points where Γ ₀₁ −Γ ₁₁ (= ΔΓ) in FIG. 8 is plotted on approximately 0, and Γ ₁₁ / Γ ₀₁ in FIG. 9 is approximately plotted on 1. Is a point. From FIG. 6, point E and point F satisfy D ₁ / D ₂ of 0.5 or more and Δ _nci / Δ _nco satisfy 0.1 or less (as described above, in FIG. 6, relative refractive index difference Δ _nci 1% the relative refractive index difference delta _nco 0%), than 7-9, at Γ ₀₁ -Γ ₁₁ (= ΔΓ) is substantially 0, it can be seen gain difference between modes is very small. On the other hand, in FIG. 6, there is a point that the gain difference between modes is not so small as in the point H or the like where D ₁ / D ₂ is 0.5 or more and Δ _nci / Δ _nco is 0.1 or less. Such a point can be said to be a state in which the cut-off wavelength of light in the LP ₂₁ mode is very small compared to the cut-off wavelength (1450 nm) in FIGS.

Figure 10 is a diagram showing a relationship between the difference [Delta] [gamma], _{LP 21} mode optical cutoff wavelength λc and _{(LP 21)} of the power gamma ₁₁ of _{LP 01} mode of light power gamma ₀₁ and _{LP 11} mode of light . In FIG. 10, D ₁ / D _{2 is set} to 0.5, Δ _{nci is set} to 0%, and Δ _nco is _set to 1%. Then, D ₂ was changed from 6.2 μm to 9.4 μm, and the cutoff wavelength of the LP ₂₁ mode light was changed as shown in FIG.

As shown in FIG. 10, it can be seen that ΔΓ becomes smaller as the cutoff wavelength of the LP ₂₁ mode light becomes longer. That is, in FIGS. 3 and 4, the cutoff wavelength of the LP ₂₁ mode light is 1450 nm, but it can be seen that ΔΓ becomes smaller if the cutoff wavelength is longer than 1450 nm. Further, FIG. 10 shows that ΔΓ becomes 0.01 or less when the cutoff wavelength of the LP ₂₁ mode light is 1430 nm or more. Therefore, as shown in FIGS. 3 and 4, if the cut-off wavelength of the LP ₂₁ mode light is 1430 nm, the tendency is not so different from that in FIGS. That is, when the cutoff wavelength of the LP ₂₁ mode light is 1430 nm, D ₁ / D ₂ is 0.5 or more, and Δ _nci / Δ _nco is 0.1 or less, ΔΓ is 0.01 or less. can do.

Next, a configuration in which the cutoff wavelength of the LP ₀₂ mode light propagating through the core 11 is made smaller than 980 nm will be described.

FIG. 11 is a diagram showing the relationship between the ratio D ₁ / D ₂ between the diameter D ₁ of the inner core 11 i and the outer diameter D ₂ of the outer core 11 o and the cutoff wavelength λc. In FIG. 11, the LP ₂₁ mode light has a cutoff wavelength of 1500 nm, the relative refractive index difference with respect to the cladding 12 of the outer core 11o is 1%, and the refractive index of the inner core 11i is equivalent to that of the cladding. It defines a D _2. Further, in FIG. 11, the above relationship is shown for LP ₁₁ mode light, LP ₂₁ mode light, LP ₀₂ mode light, LP ₃₁ mode light, and LP ₁₂ mode light. From FIG. 11, it can be seen that if D ₁ / D ₂ is 0.5 or more, the cutoff wavelength of the LP ₀₂ mode light and the cutoff wavelength of the LP ₁₂ mode are smaller than 980 nm. Therefore, if D ₁ / D ₂ is 0.5 or more, the mode of the pumping light having a wavelength of 980 nm can be set to LP ₀₁ mode, LP ₁₁ mode, LP ₂₁ mode, and LP ₃₁ mode.

Next, when D ₁ / D ₂ is 0.5, the ratio Δ _nci / of the relative refractive index difference Δ _nci of the inner core _{11 i} to the cladding 12 and the relative refractive index difference Δ _nco of the outer core _{11 o} to the cladding 12. _Δnco is changed. FIG. 12 is a diagram showing the relationship between the ratio Δ _nci / Δ _nco and the cutoff wavelength λc. As shown in FIG. 12, when the ratio Δ _nci / Δ _nco is 0.1 or less, the cutoff wavelength of the LP ₀₂ mode light and the cutoff wavelength of the LP ₁₂ mode are smaller than 980 nm. Therefore, if the ratio Δ _nci / Δ _nco is 0.1 or less, the mode of the pumping light with a wavelength of 980 nm can be set to the LP ₀₁ mode, the LP ₁₁ mode, the LP ₂₁ mode, and the LP ₃₁ mode.

That is, from FIGS. 11 and 12, when D ₁ / D ₂ is 0.5 or more and Δ _nci / Δ _nco is 0.1 or less, excitation with a wavelength of 980 nm for exciting erbium added to the outer core 11o The light mode can be LP ₀₁ mode, LP ₁₁ mode, LP ₂₁ mode, and LP ₃₁ mode.

As described above, the amplification optical fiber 10 has an LP ₁₁ mode light having a theoretical cutoff wavelength of 1565 nm or longer, and an LP ₂₁ mode light having a theoretical cutoff wavelength of 1530 nm or shorter. For example, under the condition that D ₁ / D ₂ is 0.5, Δ _nci / Δ _nco is 0, and Δ _nco is 1.0%, the above theoretical cutoff wavelength is 6 .2 μm ≦ D ₂ ≦ 9.4 μm may be satisfied.

As described above, according to the amplification optical fiber 10 of the present embodiment, it is possible to amplify the LP ₀₁ mode light and the LP ₁₁ mode light while propagating in the C band. Further, the difference between the power of the LP ₀₁ mode light propagating through the outer core 11o and the power of the LP ₁₁ mode light can be reduced. Since erbium is added to the outer core 11o, erbium is added. It is possible to reduce the difference between the power of the LP ₀₁ mode light propagating in the region where the light is transmitted and the power of the LP ₁₁ mode light. Therefore, by appropriately adjusting the inversion distribution of erbium, the difference between the gain of the LP ₀₁ mode light and the gain of the LP ₁₁ mode light can be suppressed.

In the amplification optical fiber 10 of the present embodiment, the theoretical cutoff wavelength of the LP ₀₂ mode is 980 nm or less. Therefore, when the excitation light that excites erbium propagates through the core, the mode of the excitation light is set to LP _01. Mode, LP ₁₁ mode, LP ₂₁ mode and LP ₃₁ mode. Therefore, the excitation ratios of the light in the four modes may be controlled so that the gain of the LP ₀₁ mode light and the gain of the LP ₁₁ mode light in the signal light are equal. Therefore, according to the amplification optical fiber 10 of this embodiment, the difference between the gain of the LP ₀₁ mode light and the gain of the LP ₁₁ mode light can be easily suppressed.

<Description of optical fiber amplifier>
Next, an optical fiber amplifier using the amplification optical fiber 10 will be described with reference to FIG.

  FIG. 13 is a diagram showing the optical fiber amplifier of the present embodiment. As shown in FIG. 13, the optical fiber amplifier 1 in the present embodiment is connected to an optical fiber 21 that propagates amplified signal light, an optical isolator 30 a provided in the middle of the optical fiber 21, and the optical fiber 21. WDM coupler 40a, optical fiber 22 having one end connected to WDM coupler 40a, amplification optical fiber 10 having one end connected to the other end of optical fiber 22, and one end connected to the other end of amplification optical fiber 10 Optical fiber 24, WDM coupler 40b connected to the other end of the optical fiber 24, optical fiber 25 connected to the WDM coupler 40b, optical isolator 30b provided in the middle of the optical fiber 25, and pumping light source 50 And as a main component.

The optical fiber 21 is a fu-mode fiber that propagates LP ₀₁ mode light and LP ₁₁ mode light, which are signal lights, in the C band, and a signal is transmitted to each of the LP ₀₁ mode light and LP ₁₁ mode light. Superimposed. These lights propagate through the optical fiber 21 toward the WDM coupler 40a.

  The optical isolator 30a provided in the middle of the optical fiber 21 transmits signal light propagating from the optical fiber 21 side toward the WDM coupler 40a side, and suppresses transmission of light propagating toward the opposite side. Accordingly, the light traveling in the direction opposite to the traveling direction of the signal light is prevented from entering the optical fiber 21 from the optical isolator 30a due to unnecessary reflection in the optical fiber amplifier 1 or the like.

The excitation light source 50 emits excitation light having a wavelength of 980 nm. The excitation light emitted from the excitation light source 50 individually emits LP ₀₁ mode light, LP ₁₁ mode light, LP ₂₁ mode light, and LP ₃₁ mode light. For example, except for the light in the _{LP 01} mode is _{LP 11} mode of light, _{LP 21} mode of light, the light of the LP ₃₁ mode _{LP 01} mode which is the basis of light emitted separately from the light, _{LP 11} Mode light, LP ₂₁ mode light, and LP ₃₁ mode light are separately excited. A wave phase plate may be used for excitation. Then, the excited light of each mode is individually emitted, and the light of each mode is individually incident on the WDM coupler 40a. In order to adjust the power of light in each mode, the power of light in the LP ₀₁ mode, which is the basis of the light in each mode, may be adjusted individually.

  Signal light enters the WDM coupler 40 a from the optical fiber 21, and excitation light enters from the excitation light source 50. The WDM coupler 40 a combines the incident signal light and excitation light and enters the optical fiber 22. The optical fiber 22 has the same configuration as the optical fiber 21.

In the amplification optical fiber 10 connected to the optical fiber 22, the core 11 satisfies D ₁ / D ₂ of 0.5 or more and Δ _nci / Δ _nco satisfies 0.1 or less. C-band LP ₀₁ mode light and LP ₁₁ mode light propagating from the optical fiber 21 and excitation light having a wavelength of 980 nm emitted from the excitation light source are incident on the amplification optical fiber 10. The signal light that is incident on the amplification optical fiber 10 and propagates through the core 11 has the LP ₀₁ mode light power Γ ₀₁ and the LP ₁₁ mode light power Γ _{11 in} the outer core 11 o under the condition that the core 11 satisfies. Are approximately equal. On the other hand, the modes of the pumping light that is incident on the amplification optical fiber 10 and propagates through the core 11 are the LP ₀₁ mode, the LP ₁₁ mode, the LP ₂₁ mode, and the LP ₃₁ mode based on the conditions that the core 11 satisfies. Then, erbium added to the outer core 11o is excited. Then, the excited erbium causes stimulated emission by the signal light, and the signal light is amplified.

At this time, as described above, in the outer core 11o, the power gamma ₁₁ of LP ₀₁ mode of the optical power gamma ₀₁ and LP ₁₁ mode of the light is substantially equal, the excitation of the four modes of the excitation light is adjusted respectively Therefore, the LP ₀₁ mode light and the LP ₁₁ mode light in the signal light have the same gain. In order to obtain such a gain, for example, the power of the four modes of the pumping light is the power of the LP ₀₁ mode of the signal light emitted from the amplification optical fiber 10 and the power of the light of the LP ₁₁ mode. , And the light power of the four modes of the pumping light is adjusted so that the gain of the LP ₀₁ mode light of the signal light and the gain of the LP ₁₁ mode light are comparable.

Thus, the signal light obtained by amplifying the LP ₀₁ mode light and the LP ₁₁ mode light to the same extent is emitted from the amplification optical fiber 10.

  The optical fiber 24 connected to the amplification optical fiber 10 has the same configuration as the optical fiber 22. The signal light and excess pumping light emitted from the amplification optical fiber 10 enter the optical fiber 24 and propagate through the optical fiber 24.

  The signal light and the excess pump light incident on the WDM coupler 40b from the optical fiber 24 are separated by the WDM coupler 40b. The separated excess excitation light is extinguished by the terminating device E, and the signal light enters the optical fiber 25 and propagates through the optical fiber 25.

  The optical isolator 30b provided in the middle of the optical fiber 25 transmits signal light propagating through the optical fiber 25 from the WDM coupler 40b side, and suppresses transmission of light propagating toward the WDM coupler 40b. For this reason, the signal light is transmitted through the optical isolator 30b and emitted.

According to the optical fiber amplifier 1 of the present embodiment, since the difference between the gain of the LP ₀₁ mode light and the gain of the LP ₁₁ mode light in the amplification optical fiber 10 is suppressed, Light can be emitted.

  As mentioned above, although this invention was demonstrated to the example for embodiment, this invention is not limited to these.

For example, in FIG. 2, the relative refractive index difference with respect to the clad 12 of the inner core 11i is 0%. However, if the theoretical cutoff wavelength of the LP ₀₂ mode light is 980 nm or less, the relative refractive index difference is 0%. Not limited to. However, Δ _nci / Δ _nco is preferably _set to 0.1 or less as described above.

As described above, according to the present invention, the amplification optical fiber that can easily suppress the difference between the gain of the LP ₀₁ mode light and the gain of the LP ₁₁ mode light, and an optical fiber amplifier using the same Is expected to be used in the field of fuse mode optical communications.

DESCRIPTION OF SYMBOLS 1 ... Optical fiber amplifier 10 ... Amplifying optical fiber 11 ... Core 11i ... Inner core 11o ... Outer core 12 ... Cladding 14 ... Covering layers 21, 22, 24, 25 ... Optical fibers 30a, 30b ... Optical isolators 40a, 40b ... WDM coupler 50 ... Excitation light source

Claims (7)

Comprising a core having an inner core and an outer core surrounding the outer peripheral surface of the inner core;
The relative refractive index difference between the inner core and the cladding is smaller than the relative refractive index difference between the outer core and the cladding.
Erbium is added to the entire outer core,
The theoretical cutoff wavelength of LP ₁₁ mode light is 1565 nm or more,
LP ₂₁ mode light has a theoretical cutoff wavelength of 1530 nm or less,
LP ₀₂ mode light has a theoretical cutoff wavelength of 980 nm or less ,
When the inner core diameter is D ₁ and the outer core outer diameter is D ₂ , D ₁ / D ₂ is 0.5 or more,
When the relative refractive index difference between the inner core and the clad is Δ _nci and the relative refractive index difference between the outer core and the clad is Δ _nco , Δ _nci / Δ _nco is 0.1 or less. An amplification optical fiber characterized by the above. Amplifying optical fiber according to claim 1, D ₁ / _{D 2} is characterized in that it is 0.6 or more. Amplifying optical fiber according to claim 1 or 2 D ₁ / _{D 2} is characterized in that it is 0.8 or less. The optical fiber for amplification according to any one of claims 1 to 3 , wherein Δ _nci / Δ _nco is 0 or more. The optical fiber for amplification according to any one of claims 1 to 4 , wherein a theoretical cutoff wavelength of LP ₂₁ mode light is 1430 nm or more. 6. The amplification optical fiber according to claim 5 , wherein a theoretical cutoff wavelength of the LP ₂₁ mode light is 1450 nm or more. An optical fiber for amplification according to any one of claims 1 to 6 ,
An excitation light source that emits excitation light having a wavelength of 980 nm that is incident on the core;
An optical fiber amplifier comprising:

Images